Process for making a multicolor fluorescent screen



Aug. 24, 1954 E; MICHAELS Filed Jan. 18 1951 Fig 1 EDWARDL.

MICHAELS ms ATJDRIIEY Patented Aug. 24, 1954 PROCESS FoRMAKm FLUORESCEN G A mct'ricotoa T SCREEN Edward L. Michael s, River rarest. Illa; assignoi' to The Remand Illinois Corporation, a corporation of App'ii'catmfi January 18, 1951, Serial No. 206,589

1 Claim.

This invention relates toa new and improved process for making multi-color fluorescent screens for cathode-ray tubes used inthe reproduction of televised images in natural color. V

It is known that televised images may be reproduced in natural color by employing a cathoderay tube having a multi-colorfluor'escent screen of the type comprising a plurality oi interspersed groups of phosphor elements having different spectral response to electron bombardment. In one form of such a tube, the screen comprises three interspersed groups of phosphor elements respectively exhibiting maximum spectral response at the three primary colors, red, blue and green. A screen of this type is placed behind a parallax grid having an aperture pattern corresponding to thespace distribution of one of the groups of phosphor elements onthe screen, and an electron beam, which is intensit'y-modulated in accordance with the respective componentcolor signals, is caused to approach the parallax grid from diflerent angles for each of the three component colors so that only one of the three color-groups of phosphor elements is excited durmg each scansion of the beam. Alternatively three separate electron beams may be employed, one for each color, these beams being gated in accordance with a received color-synchronizing signal in order to prevent color mixing or contamination. 7

It is known that a screen of this type, comnionly referred to as a dot-interlace screen, may be formed by successively screening suspensions of the receptive phosphors through a suitably pre pared photographic stencil, the stencil being laterally displaced with respect to the screen surface between application of the several phosphors to provide the desired dot-interlace pattern. However, it has not been previously been thought important to control thesequence in which the phosphors are applied; indeed it has been thought that the sequence of applying the phosphors is of little or no consequence in determing the characteristics of the finished screen.

It is an important object of the invention to provide anew and improved process of preparing a multi-color fluorescent screen in such amanner as to eiTect an improvement in the overall brightness of the screen. t v

The present invention is based on the recognition that the luminance characteristicsof the several phosphors employed are generally not identical. For example, the commonly employed green-responsive phosphors exhibit a higher luminance under electron bombardment than those conventionally used for red and blne re sponse. In accordance with the invention, the phosphor of highest luminance is arranged on the supporting plate in a regular pattern of discrete elements individually having a predetermined area, and the one or more phosphors of lower luminande are arranged on the supporting late in regular patterns of similar discrete ele- Inents individually having an area larger than that of each element of the high-luminance phosphor The smallest elements are thus composedof the phosphor having the highest brightnes s density; while the low-luminance phosphors possess a low er brightness density under electron bombardment, thearea of each phosphor element is larger so that the integrated brightness approaches that of the smaller high-luminance phosphor elements. ,1

p Also in accordance with the invention, an improved mum-com fiuorescentsc'reen of this type may be prepared by screening the respective phosphors onto the supporting plate in a predetermined sequence related to the luminance characteristics of the phosphors. The improved method comprises placing a stencil, having an aperture pattern oorresponding to the desired area distribution of each of the phosphors, in contact with the supporting plate. The phosphor having the highest luminance in response teencuop bombardment is screened through the stencil onto the supporting plate to apply this phosphor to the plate in the desired area distribution. The stencil is then lifted from the supporting plate and laterally shifted to a position in which the aperture pattern is in registration with an uncoaten portion of the supporting plate. The stencil is then placed over the elements of the previously applied phosphor and is thus supported in a position slightly spaced from the unoo'ated portion or the supporting plate by a distanee determined by the thickness of the previously applied phosphor coating. The second phosphor; which has a lower luminance response. is then screened through the stencil onto the uncoatetl portion of the supporting plate. When the stencil is finally removed there will remain interspersed elements of the different phosphors on the supporting plate with the individual elemerits of the second phosphor of larger area than the corresponding individual elements of the previously applied phosphor. In this manner, the desired improvement in over-all brightness of the multi-color screen may be obtained without any increase in the cost of preparing the screen since the only required modification in the known process is in the sequence of applying the several phosphors.

As used throughout the specification and claims, the term luminance is a measure of the brightness of a phosphor response to electron bombardment under eonditions corresponding to actual operation of the tube in a color television receiver. Luminance is defined as the luminous flux in lumens per unit solid angle, the commonly used unit being the foot-lambert, which is equivalent to 0.0003426 candle per square centimeter.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in connection with the accompanying drawing, in the several figures of which like reference numerals indicate like elements, and in which:

Figure 1 is an enlarged plan view, partly broken away, of a photographic stencil and metal screen which may be employed in carrying out the process of the present invention;

Figure 2 is a schematic edge view, partly broken away, illustrating the use of the stencil and screen of Figure 1 in applying the screen phosphors to a supporting plate; and

Figure 3 is a greatly enlarged plan view of a portion of a multi-color fluorescent screen constructed in accordance with the invention.

As shown in Figure 1, a stencil i and a metal or silk screen I i may be supported in close mutual contact by means of a frame 12. The stencil may be prepared in accordance with a known process involving the preparation of a photographic negative from which is made a contact print on a sensitized sheet of carbon tissue or other transparent material coated with sensitized material which has the property of being soluble until such time as it is exposed to light. After exposure to light through the photographic negative, the carbon tissue is developed in warm water or other suitable solvent to dissolve out the unexposed portions of the material. The resulting stencil l0 comprises a regular pattern of apertures, preferably circular in shape and or" equal areas, corresponding to the desired distribution of an individual screen phosphor corresponding to one of the primary colors. In the illustrated embodiment, the circular holes or apertures are spaced horizontally by a distance equal to substantially twice the diameter of an individual aperture and vertically by a distance substantially equal to the diameter of an individual aperture, and the even-numbered horizontal rows are laterally displaced with respect to the odd-numbered rows by a distance equal to one-half the horizontal spacing between adjacent apertures. The individual apertures may be of a diameter of the order of 0.006 to 0.008-inch, for example, andthe metal or silk screen i I, which is employed merely for ceonvenience, may be from two hundred to three hundred mesh.

The different screen phosphors which are to be applied to the supporting plate are dispersed respectively in suitable vehicles, such as ethyl nitrocellulose, to about the consistency of ordinary enamel paint. The phosphor suspensions are sequentially printed through the holes or apertures in the stencil l0 and through the corresponding portions of the silk or metal screen I 1 onto a supporting plate [3 constructed of glass or other suitable fluorescent screen-receiving material.

In carrying out this part of the process, one of the phosphor suspensions is passed through the stencil l0 and the silk or metal screen I I by means of a squeegee It or the like, thus applying the phosphor to the supporting plate 13 through the openings in the stencil onto small areas of the 4 supporting plate which correspond to the aperture pattern of the stencil.

After applying the first screen phosphor in this manner, the supporting plate and the stencilscreen are dried and the stencil-screen or the supporting plate is displaced laterally with respect to the other by a distance substantially equal to the diameter of one of the circular apertures, or to one-third the center-to-center spacing between two adjacent holes in a horizontal row. The second phosphor suspension is then applied to the supporting plate in the same manner as the first. After drying the stencil-screen and the supporting plate again, the stencil-screen or supporting plate is once more displaced laterally by a distance corresponding to the diameter of one of the circular holes or to one-third the center-to-center spacing between two adjacent holes in a horizontal row in the stencil, and the third phosphor suspension is applied to the supporting plate. Owing to the spacing of the apertures in the stencil, the three successive phosphor applications substantially completely cover the surface of the supporting plate with the three phosphors arranged in interspersed regular patterns of similar non-overlapping discrete elements; viewed in another way the screen comprises a repetitive pattern of three-component clusters of phosphor materials each of which comprises a triangular arrangement of red, blue and green phosphor dots.

While numerous phosphor materials for providing the desired spectral response to each of the three primary colors are well known in the art, a screen having the desired properties may be provided by employing zinc silicate as the green phosphor, magnesium-activated calcium silicate as the blue phosphor, and cadmium borate as the red phosphor. Experimentation has revealed that of these three phosphors, zinc silicate (the green phosphor) exhibits a materially higher luminance in response to electron bombardment than do magnesium-activated calcium silicate and cadmium borate (the blue and red phosphors). Measurements made on a screen containing equal-area color-groups and corrected for the spectral selectivity of the human eye indicate that the luminance of the green phosphor may be 1.5-2 times that of the red phosphor and 3-4 times that of the blue phosphor. In order to improve the uniformity of the over-all brightness of the finished screen, therefore, that portion of the area of the screen surface which is coated with the green phosphor is made smaller than those coated respectively with the blue and red phosphors. This relation is obtained whether the respective areas are measured with respect to individual dot elements or in the aggregate. The green phosphor, being of highest luminance, exhibits the greatest brightness density in response to electron bombardment but covers a smaller portion of the screen surface than either the red or the blue phosphor, with the result that the integrated brightness may be substantially equalized for red, blue and green. For best results, the areas should be related as substantially the inverse of the ratio between the luminances of the phosphors, altough a noticeable improvement is obtained even if the area relationship falls short of this optimum condition.

This desired result may be obtained by observing one added instruction in carrying out the process described in connection with Figures 1 and 2. In accordance with the invention, the highest luminance phosphor (the green phosphor) is applied through stencil ill and screen II to the supporting plate [3 before either of the other phosphors. Since the sill: or metal screen II is pressed into intimate contact with the surface of the supporting plate by squeegee i l in its passage across the top of the stencil, the individual phosphor dots applied to the supporting plate are identically equal in size to the circular apertures in stencil i 0. However, when the stencil-screen assembly is positioned on the plate for the application of the second color phosphor, intimate contact between the metal screen and the supporting plate is prevented by the dried dot-like deposits of the first screen phosphor. Consequently, a certain amount of spreading occurs between the stencil and the surface of the supporting plate during the application of the second color phosphor, and the dot elements of the second phosphor are somewhat larger in diameter than the circular apertures in the stencil. The third phosphor is deposited in still larger dot elements since the stencil-screen is farther separated from the supporting plate by the first two groups of phosphor elements than by the first group alone.

Figure 3 is a greatly enlarged view of a portion of a multi-color fluorescent screen produced in accordance with the above described process. The fluorescent screen comprises three interspersed groups of discrete phosphor elements on supporting plate l3, the red, blue and green phosphor elements being designated R, B and G respectively. Since the green phosphor in the illustrated example is of the highest luminance, it is the first to be applied and the green phosphor elements, G, are of smaller area than the red and blue elements, R and B. If the blue phosphor is the second to be applied to the supporting plate, the blue elements are of smaller area than the red elements. As a practical matter, the difference in size between the red and blue elements is much smaller than that between the green elements and either the red or blue elements, but this is of small consequence since the red and blue phosphors'are of nearly equal luminance.

Afterdeposition of the three groups of color phosphors and subsequent drying and baking in a conventional manner, the fluorescent screen may be assembled within a cathode-ray tube envelope (not shown) in any convenient manner. For example, the fluorescent screen and a suitable parallax grid may be mounted in a common frame which is supported by brackets extending inwardly from the wall of a metal cone member in a suitable predetermined space relation with an electron gun housed within the constricted neck portion of the envelope. The envelope may then be sealed and evacuated in accordance with well known procedures to complete the fabrication of the tube.

While the present invention has been shown and described in connection with a multi-color fluorescent screen of the dot-interlace type, the principles of the invention are equally adaptable to other types of multi-color screens, as for example, the strip-type in which the difierent color phosphors are arranged in interspersed groups of strip elements extending longitudinally for the full width of the screen. To obtain the advantages of the invention, it is only necessary that the phosphor with the highest luminance be applied in such a way that its area, measured either for an individual element or in the aggregate, is less than the correspondingly measured area for the phosphor or phosphors of lower luminance. It is may be made, and it is also contemplated that this desired condition may be obtained by means of other processes, as for Thus, the present invention provides a new and improved process for making a multi-color fluorescent screen afiording a substantial improvement in brightness uniformity as compared with screens of a similar type prepared in accordance with known procedures. Moreover, the improvement in brightness uniformity may be achieved at no added expense in the-processing of the tube by merely applying the respective color phosphors in a sequence determined by their luminance characteristics, rather than in a haphazard sequence as has been done in the past.

While a particular embodiment of the present invention has been shown and described, it is apparent that various changes and modifications therefore contemplated in the appended claim to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

The method Of forming a multi-color fluorescent screen of the type comprising at least two phosphors exhibiting different color-radiation characteristics in response to electron bombardment, one of said phosphors having a higher luminance in response to electron bombardment than the other, said phosphors being arranged in interspersed similar patterns of discrete elements on a single supporting plate, which method comprises: placing in contact with said supporting plate a stencil having an aperture pattern corresponding to the desired area distribution of each of said phosphors; screening said one phos- I phor through said stencil onto said supporting plate to apply said one phosphor to said plate in said desired area distribution; lifting said stencil from said supporting plate; laterally shifting said stencil to a position in which said aperture pattern is in registration with an uncoated portion of said supporting plate; placing said stencil over the elements of said one phosphor coated on said supporting plate, whereby said stencil is supported in a position slightly spaced from said uncoated portion of said supporting plate; screening said other phosphor through said stencil onto said uncoated portion of said supporting plate; and removing said stencil to leave interspersed elements of said diiferent phosphors on said supporting plate with the indivdual elements of said other phosphor of larger area then the corresponding individual elements of said one phosphor.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,310,852 Leverenz Feb. 9, 1943 2,429,849 Somers Oct. 28, 1947 2,442,961 Ramberg June 8, 1948 2,485,607 Kasperowicz Oct. 25, 1949 2,508,267 Kasperowicz May 16, 1950 2,543,477 Sziklai et al Feb. 27, 1951 2,590,018 Koller et a1 Mar. 18, 1952 FOREIGN PATENTS Number Country Date 866,065 France Mar. 31, 1941 

